This invention relates to a catalytic technique for cracking heavy petroleum stocks and upgrading light olefin gas to heavier hydrocarbons. In particular, it provides a continuous integrated process for oligomerizing olefinic light gas byproduct of cracking to produce C.sub.5.sup.+ hydrocarbons, such as olefinic liquid fuels, aromatics and other useful products. Ethene, propene and/or butene containing gases, such as petroleum cracking light gas from a fluidized catalytic cracking unit may be upgraded by contact with a crystalline medium pore siliceous zeolite catalyst.
Developments in zeolite catalysis and hydrocarbon conversion processes have created interest in utilizing olefinic feedstocks for producing C.sub.5.sup.+ gasoline, diesel fuel, etc. In addition to basic chemical reactions promoted by zeolite catalysts having a ZSM-5 structure, a number of discoveries have contributed to the development of new industrial processes. These are safe, environmentally acceptable processes for utilizing feedstocks that contain lower olefins, especially C.sub.2 -C.sub.4 alkenes. Conversion of C.sub.2 -C.sub.4 alkenes and alkanes to produce aromatics-rich liquid hydrocarbon products were found by Cattanach (U.S. Pat. No. 3,760,024) and Yan et al (U.S. Pat. No. 3,845,150) to be effective processes using the zeolite catalysts having a ZSM-5 structure. U.S. Pat. Nos. 3,960,978 and 4,021,502 (Plank, Rosinski and Givens) disclose conversion of C.sub.2 -C.sub.5 olefins, alone or in admixture with paraffinic components, into higher hydrocarbons over crystalline zeolites having controlled acidity. Garwood et al. have also contributed to the understanding of catalytic olefin upgrading techniques and improved processes as in U.S. Pat. Nos. 4,150,062, 4,211,640 and 4,227,992. The above-identified disclosures are incorporated herein by reference.
Conversion of lower olefins, especially propene and butenes, over HZSM-5 is effective at moderately elevated temperatures and pressures. The conversion products are sought as liquid fuels, especially the C.sub.5.sup.+ aliphatic and aromatic hydrocarbons. Product distribution for liquid hydrocarbons can be varied by controlling process conditions, such as temperature, pressure, catalyst activity and space velocity. Gasoline (C.sub.5 -C.sub.10) is readily formed at elevated temperature (e.g., up to about 400.degree. C.) and moderate pressure from ambient to about 5500 kPa, preferably about 250 to 2900 kPa. Olefinic gasoline can be produced in good yield and may be recovered as a product or fed to a low severity, high pressure reactor system for further conversion to heavier distillate-range products.
Recently it has been found that olefinic light gas can be upgraded to liquid hydrocarbons rich in olefins or aromatics by catalytic conversion in a turbulent fluidized bed of solid medium pore acid zeolite catalyst under effective reaction severity conditions. Such a fluidized bed operation typically requires oxidative regeneration of coked catalyst to restore zeolite acidity for further use, while withdrawing spent catalyst and adding fresh acid zeolite to maintain the desired average catalyst activity in the bed. This technique is particularly useful for upgrading FCC light gas, which usually contains significant amounts of ethene, propene, C.sub.1 -C.sub.4 paraffins and hydrogen produced in cracking heavy petroleum oils or the like.
Economic benefits and increased product quality can be achieved by integrating the FCC and oligomerization units in a novel manner. It is a main object of the present invention to further extend the usefulness of the medium pore acid zeolite catalyst used in the olefinic light gas upgrading reaction by withdrawing a portion of partially deactivated and coked zeolite catalyst and admixing the withdrawn portion with cracking catalyst in a primary FCC reactor stage. Prior efforts to increase the octane rating of FCC gasoline by addition of zeolites having a ZSM-5 structure to large pore cracking catalysts have resulted in a small decrease in gasoline yield and increased light olefin by product.